A conventional Global Positioning System (GPS) receiver may provide very accurate position location information on the surface of the Earth. In general, the GPS receiver uses a group of more than two dozen GPS satellites that broadcast precise timing signals (via electromagnetic waves) by radio to electronic GPS receivers, allowing the receivers to accurately determine their location (i.e., latitude, longitude, and altitude) in real time. However, a GPS receiver generally may not be used in underwater environments, because of an extremely high attenuation of electromagnetic waves in sea water.
In contrast to electromagnetic waves, sound signals (acoustic waves) may propagate in water with relatively minimal attenuation. Accordingly, high frequency acoustic techniques are widely used for underwater imaging, as well as for location and positioning of underwater objects mainly for comparatively short ranges (a few km). It is desirable to accurately determine the position of an object over long range distances (a few tens of km) in shallow water channel environments (e.g., where the depth is about 200 m or less), such as in continental shelf regions.